The present invention relates to an electrical fuse circuit in which part of a fuse device is blown by passing a current to change a resistance value of the fuse device from a low level to a high level, thereby performing programming.
Conventionally, electrical fuses have been used as programming devices for solving redundancy and the like in a memory in a system LSI.
According to a known technique, a series circuit including a fuse device and a MOS transistor is connected between a program power source and a ground potential on a system LSI. The program power source is used also as a cell power source (VDD_IO, for example, 3.3 V) in an input/output cell region of the system LSI. This is because VDD_IO, which is a higher voltage than a voltage of a power source (VDD_CORE, for example, 1.2 V) of a normal logic circuit is used in order to ensure a large amount of a current flowing for programming a fuse device. Moreover, to perform programming, a control circuit for selecting a fuse device which is to be programmed is needed. For the control circuit, the power source VDD_CORE of a normal logic circuit is used. Therefore, an output at a VDD_CORE voltage level of the control circuit is converted into a signal at a VDD_IO voltage level by a level shifter and the MOS transistor is made conductive by the signal obtained through the conversion, thereby performing programming (see U.S. Pat. No. 7,203,117B2).
However, when a power source of a system LSI is turned ON, there is no certainty that voltages from a VDD_IO power source and a VDD_CORE power source are applied at the same time. For example, when it is assumed that VDD_IO is applied before application of VDD_CORE, a voltage from a power source is applied to a fuse device but not to the control circuit. In such a case, an output of the level shifter becomes unstable, so that the MOS transistor might become conductive and a fuse device might be misprogrammed. This also occurs when the power source is turned OFF.
In recent years, with the popularization of portable commercial products, a technique in which the function of cutting off a voltage from a power source or a ground for each block while equipment is in a halt state is provided in a circuit in a system LSI to suppress a leakage current therein has been used. Also in such a case, for example, if in cutting off a voltage from a power source or a ground, cut-off of a voltage of the power source or the ground in a fuse device side is slightly delayed, an output of a level shifter becomes indefinite, so that a MOS transistor is made conductive and a fuse device is misprogrammed. This also occurs when the power source or the ground is recovered from a cut-off state.